Many details have emerged following Monday’s shooting rampage at the Washington Navy Yard, including the fact that four Fairfax County residents were among the 12 people killed.

One of those was Lorton resident Michael Arnold, a retired Navy officer who had once been posted at Pearl Harbor. Arnold, 59, was a senior civilian contractor who had put his career’s worth of experience into designing and overseeing the construction of better naval warships for the next generation, his co-workers told the Washington Post.

He was “an institution” who knew the Navy’s America-class amphibious assault ships as well as anyone, said Capt. Mark Vandroff, a colleague, adding that Arnold was the Navy’s go-to guy in negotiations over any purchase for America-class ships.

“Nobody knew those ships like him. He was the guy you depended on every time you went to talk to the contractor, to make sure you were getting what you had ordered.” Vandroff said. “Michael’s family will grieve for him, as a lost husband and lost father, but the Navy will also grieve. It has lost a tremendous human intellectual asset and just a wonderful man.”

Mary Francis Knight, 51, of Reston worked for the Naval Sea Systems Command, and had been living in Reston for about the last five years. She spoke every day to her younger sister or her daughters, enjoyed working out and was a practicing Catholic, said her brother-in-law Theodore Hisey, a family spokesman.

She was born in Fayetteville, N.C., the middle child of a Green Beret who was an instructor at Fort Bragg, and a stay-at-home mother, Hisey said. She also had recently become an adjunct assistant professor at Northern Virginia Community College in Loudoun and Annandale. College officials said she was supposed to teach a computer class this summer, but the class was canceled because of low enrollment. Her fall semester classes started Aug. 21. She taught spreadsheet software Monday nights at the Loudoun campus, and software design Thursday nights in Annandale.

Martin Bodrog, 54, lived in Annandale with his wife, Melanie, whom he’d met in Newport, R.I., where she was serving as a Naval nurse and he was an instructor at Naval Surface Warfare School, close friend Jeffrey Prowse told the Washington Post. Bodrog was born in Woodbury, N.J., and graduated from the Naval Academy in 1981. Officially, he served 22 years, retiring as a Surface Warfare Officer. But he never really left the service, finding a second, civilian career at the Pentagon, where he oversaw the design and procurement of the amphibious war ships used to ferry U.S. Marines and their supplies around the world.

“His expertise and experience in amphibious operations allowed Marty to make lasting contributions to the success of the Navy-Marine Corps Team,” said Prowse.

Gerald Read, 58, of Alexandria served at Fort Belvoir during the wars in Iraq and Afghanistan, working with the U.S. Army Materiel Command, supervising efforts to supply and maintain forces deployed overseas. In recent years, he turned to civilian work at the Navy Yard, managing security risks related to information and data. Read was passionate both about family life and his job, “totally reliable, really, really solid,” his wife told the Washington Post. She had no details about what had unfolded before her husband was killed Monday, but given his nature, she said, “I’m sure he was right in the middle of it.”

A day after the shootings in Building 197 of the Navy Yard, she recalled her husband’s love of reading — he was a Civil War buff — and his bond with their daughter, Jessica, and his three grandchildren.

“He was a fine family man and a good friend,” said Jim Miles, his next-door neighbor. “I’m just devastated that he’s gone.”

It has also been revealed that the shotgun used by suspected shooter Aaron Alexis, a 34-year-old defense contract employee and former Navy reservist, was legally purchased at the Sharpshooters Small Arms Range and gun store in Lorton. Some questions still remain, however, as to when it was actually purchased. According to the store’s attorney, J. Michael Slocum, on Sunday, Alexis went to the store, rented a rifle, bought some ammunition and practiced on the firing range. He then purchased a Remington 870 shotgun and two boxes of ammunition containing 24 shells.

But Virginia State police said that an employee at the gun store electronically submitted Alexis’ background check to the Virginia Firearms Transaction Center at 3 p.m. Saturday, where it was approved four minutes later.

“It was a lawful purchase of a shotgun made in Virginia,” said FBI spokeswoman Valerie Parlave during a press conference on Tuesday, adding that Alexis is believed to have walked into the Navy Yard armed with only that shotgun.

“At this time, we believe that Mr. Alexis entered Building 197 at the Navy Yard with a shotgun. We do not have any information at this time that he had an AR-15 in his possession,” said Parlave. “We also believe Mr. Alexis may have gained access to a handgun once inside the facility and after he began shooting. As previously mentioned, Mr. Alexis had legitimate access to the Navy Yard as a result of his work as a contractor, and he utilized a valid pass to gain entry to the building.”

“Aaron Alexis was an employee of a company called ‘The Experts,’ a subcontractor to an HP Enterprise Services contract to refresh equipment used on the navy marine corps intranet network,” Hewlett Packard said in a statement.

The Experts is based in Alexandria. In its own statement, the company offered its condolences to victims’ families.

“The Experts would like to express our deepest condolences and sympathies regarding the incident that occurred at the DC naval yards,” the statement said.” We are actively cooperating with the FBI and other authorities in relation to the investigation on the suspect. Any additional information we have will be shared accordingly.”

Metropolitan Police Department officials said they first received a call about the shooting at 8:23 a.m. Monday for the report of gunshots in 1333 Isaac Hull Ave., on the grounds of the Washington Navy Yard. Upon their arrival, police said they were directed to Building 197, and told there was an “active shooter.” The MPD Active Shooter team then assembled, and began a search of the building for victims and suspects. Police said Tuesday that Alexis was soon located inside the building and after firing multiple rounds at police, was shot to death by officers just over 30 minutes after police arrived. Police said many questions still remain unanswered and that the shooting will continue to be under investigation for quite some time.

“We are not going to stop until we get answers,” said U.S. Attorney Ronald Machen during a press briefing on Tuesday. “We will run down every lead.”